The solar system was supposed to have more gas giants — at least that’s what previous attempts to model the solar system’s emergence said. So how’d we end up with only Jupiter and Saturn? A new paper published today in Nature attempts to answer why, and it could change the prevailing view of how all of the planets formed.

Older theories were based on the theory of “pebble accretion,” which explains how the gas giant cores formed so quickly. When the Big Bang happened, dust and gas blew outward in a swirling disk, with the Sun at the middle, eating up everything in sight. It’s thought that the gas giants started off as pebbles, which formed as tiny leftover grains of dust began sticking to each other, held together by static electricity. As multiple “pebbles” collided with each other, swept up by gusts of gaseous winds, they eventually coalesced to form clumps over 100 km in diameter. At this size, the colliding clumps were bound together by gravity, and they eventually grew to become true gas giant cores — with masses equal to about 100 Earths.

The problem is, this model predicted that way more than two cores would snowball into existence. Simulations estimated that dozens or even hundreds of Earth-sized bodies would form. Until now, scientists haven’t been able to figure out how we ended up with just two big cores.

Hal Levison, the planetary scientist at the Southwest Research Institute in Boulder, Colorado who co-authored the Nature paper, originally set out to kill this theory, but he ended up refining it instead. He tweaked the speed at which the pebbles formed, giving the biggest ones time to fling their competitors out of orbit. The new simulation’s results finally got it right: Two rocky cores, which would end up as Jupiter and Saturn, and two icy ones, forming the cores of Uranus and Neptune.

The problem of “too many giants,” which has plagued scientists for years, can finally be laid to rest. “After many years of performing computer simulations of the standard model without success,” said co-author Dr. Martin Duncan in a press release, “it is a relief to find a new model that is so successful.”

Photos via NASA.gov